Video: X-37B Space Plane Lands After 2 Years in Orbit

The U.S. Air Force's mysterious X-37B space plane landed this week after a record of nearly two years in orbit, yet government officials still haven't commented as to the nature of its mission. The 'orbital test vehicle' is the third such OTV mission, and orbited the Earth for 674 days.

Naturally, the lack of explanation regarding the mission's nature has led to rampant speculation and even wild conspiracy theories. The Air Force calls the X-37B the "newest and most advanced re-entry spacecraft," of which it currently has two. The combined time in orbit by both OTVs, which look a lot like NASA's discontinued space shuttles, is currently 1,367 days. This latest mission began on Dec. 12, 2012 and was originally only expected to last around nine months.

The Air Force has released statements regarding the mission, however, they've largely been vague and haven't revealed any specifics about what exactly the X-37B does when it is orbiting the Earth.

"Technologies being tested in the program include advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, and autonomous orbital flight, re-entry and landing," the Air Force said in a statement.

Some have speculated that the X-37B is either part of an advanced spying apparatus or some kind of space weapon. Going even further, some have also put forth the idea that the OTV is spying on foreign satellites or China's space station.

The previous OTV mission lasted 469 days and also landed at Vandenberg on June 16, 2012 and was flown by the second of the two orbiters, while this most recent mission featured the first of the two orbiters after it was refit following its previous mission.

There's no current word on how long the Air Force is going to continue the program, but more missions are expected. "Officials anticipate multiple missions will be required to satisfy the test program objectives, but the exact number of missions has not been determined," the Air Force said.

Both X-37B OTVs measure only 29 feet and three inches in length with a wingspan of 14 feet and 11 inches and weigh a reported 5.5 tons.